1. Field of the Invention
The present invention relates to switching power supplies, and more particularly, to an AC-to-DC power factor correcting power converter comprising a boost power converter coupled to a DC-to-DC power converter where both converters are driven by the same signal derived from the output voltage of the power converter.
2. Description of the Prior Art
Many electronic devices, such as computers and many household appliances, require one or more regulated DC voltages. The power for such electronic devices is ordinarily supplied by power converters that convert an AC line voltage into the regulated DC voltages required by the devices.
Electrical power converters commonly include a rectifier circuit which converts the AC line voltage to an unregulated DC voltage, also known as a rectified line voltage, and a DC-to-DC converter for converting this unregulated DC voltage into one or more regulated DC output voltages. If a simple rectifier circuit is used, such power converters commonly draw high currents near the peak of the AC voltage cycle, and substantially zero current around the zero-crossing points of the voltage cycle. Thus, the input current drawn by the converter has a highly non-sinusoidal waveform with correspondingly high harmonic content.
As is known in the electrical power art, current harmonics above the fundamental frequency of the voltage do not contribute to the power drawn from a typical AC voltage source, with the result that the actual or true power drawn by the power supply is lower than the apparent power drawn.
The distinction between apparent power and true power is important because power supplies are rated according to the apparent power drawn rather than the true power drawn. As a basis of comparison, the true power and apparent power drawn by a device are divided to form a ratio called the xe2x80x9cpower factor.xe2x80x9d Power factors less than about 80 percent can pose barriers to the performance or improvement of many types of electronic devices that operate on direct current, including such devices as personal computers, minicomputers, and appliances using microprocessors. For example, the high current peaks associated with lower power factors can cause circuit breakers on the AC line to trip, which limits system design in terms of the functional load it places on the AC line. Additionally, the harmonics associated with the high, non-sinusoidal current peaks often result in power-line distortion, noise, and electromagnetic interference (EMI). In general, improving the power factor of the device reduces the harmonic content and electromagnetic noise.
To address these problems, many power supplies include power factor correction circuitry that is designed to raise power factors and eliminate harmonic distortion. Such circuits are often referred to as power factor correction circuits (abbreviated xe2x80x9cPFCxe2x80x9d). Power factor correction circuits generally rectify the AC line voltage and produce an unregulated DC voltage (referred to herein as the xe2x80x9cPFC voltagexe2x80x9d) in a manner that has a relatively high power factor within a given range of AC line voltages. A switching power converter then converts the PFC voltage into the required regulated voltages.
The International Electrotechnical Commission (IEC) has set standards specifying certain requirements for AC-to-DC power converters. Specifically, the IEC-1000-3-2 standard requires that the harmonic components of the input current be below a certain level. Accordingly, it is desirable to provide power factor correction for AC-to-DC power converters in order to achieve a low input current harmonic content in the AC supply, and, equivalently, a higher power factor.
One known embodiment for providing power factor correction involves using an AC-to-DC converter followed by a DC-to-DC converter where the former is a boost converter and the latter is a flyback converter. In order to provide power factor correction with this topology, the drive signal of the boost converter must be controlled to force the current flowing through the inductor in the boost converter to follow the sinusoidal waveform of the rectified input AC signal. However, conventional control techniques used to achieve this result suffer from a number of drawbacks. The predominant drawback is that a complex control circuit is needed, as well as bulky and heavy filter components to filter ripple current and to meet the EMI specification of the power supply. The result is a high cost control circuitry and the need to use more printed circuit board (PCB) space, which in turn contributes to higher fabrication costs. The overall size of the AC-DC converter is also increased.
A conventional boost converter generally comprises an inductor which is coupled between a source of AC power (e.g., a rectifier producing an unregulated AC voltage) and a switch. The switch is preferably a MOSFET transistor, which is in turn coupled in parallel with a series combination of a rectifier and an output filter capacitor across which the load is connected. The output capacitor is selected to be large in order to ensure that the load receives a substantially constant DC voltage. This constant DC signal appears across the load and is greater than the peak sinusoidal value of the input AC voltage.
One well known control method for providing power factor correction in a boost converter is to set the duration of the ON state (e.g., a time Ton out of a period T) of the FET switch to a constant value. The constant duration of Ton is predetermined by certain operating conditions, such as the input voltage, output voltage and output current of the boost converter. Energy is stored in the boost converter""s inductor during this time Ton. Additionally, certain circuit parameters, such as the value of the boost inductor, affects the duration of Ton. When the FET switch is switched to its OFF state for a certain time period (e.g., Toff), the polarity across the inductor reverses so that the energy that was stored in the inductor during Ton is transferred via the diode to the output capacitor. A constant DC voltage, VDC, appears across the capacitor and has the following relationship with respect to the rectified input AC voltage:                               V          DC                =                              V            in                                1            -                                          T                on                            /              T                                                          (        1        )            
While this control method ensures that the boost converter operates at the boundary of continuous and discontinuous modes of operation, it suffers from three drawbacks. First, because the duration of Ton is fixed, the ripple current passing through the inductor is large. Accordingly, a bulky and heavy Electromagnetic Interference (EMI) filter is required to filter out this ripple current in order to meet the EMI specification of the power supply. Second, the switching frequency of the transistor in the boost converter must be varied in order to regulate the constant DC voltage VDC. Such variable switching frequency control is usually undesirable for applications involving telecommunications because of possible interference. Third, because the control chip and accompanying current sensing circuit are necessary in order to provide power factor correction, the inclusion of these components occupies a significant portion of the PCB area and results in increased size and cost of the power supply.
Another well known control method for providing power factor conversion in a boost converter and for producing a constant DC voltage that is substantially free from distortion is conventionally known as the average current mode control method. With this technique, the boost converter operates in a continuous conduction mode. In particular, to obtain a high power factor, the average value of the current passing through the inductor in the boost converter is sensed and forced to follow in phase the rectified sinusoidal waveform of the input AC voltage Vin. In this control method, a multiplier is needed to generate the reference current for the boost inductor current. The average value of the boost inductor current must be sensed to achieve average current control.
A key drawback of the average current control method is the high cost of the control circuit.
What is needed is improved power factor correction in a power converter whereby the boost and DC-DC converters are driven with a control method that overcomes the above described drawbacks.
The present invention is directed to a power supply system having power factor correction. The system includes an input rectifier for generating a rectified input AC voltage from an AC power source for two stages of power conversion. The first stage is a boost converter. It is coupled to the input rectifier and converts the rectified input AC voltage into a substantially constant first DC voltage. The boost converter includes a first switch. The second stage converter is a DC-to-DC converter and is coupled to the output of said boost converter. The second converter converts the first DC voltage to a second voltage, the output voltage of the power supply system. The second converter regulates the output voltage to a desired level. The second converter includes a second switch. The power supply system also includes a controller for providing feedback control as a function of the output voltage, said controller generating a drive signal for said first and second switches so as to cause said switches to be switched on and off simultaneously.
Accordingly, it is an object of the present invention to provide a power supply system for providing power factor correction in a cost efficient manner. Cost efficiency of power factor conversion is achieved because the present invention does not require current sensing circuitry to sense the boost converter""s inductor current, nor the corresponding additional control circuitry for the boost converter as required in the prior art. PCB space is also reduced.
It is another object of the present invention to reduce EMI by fixing the switching frequency of the controller""s drive signal at a constant value. By maintaining a constant frequency, the low frequency ripple component appearing at the output voltage is substantially reduced. This is essential for power supplies used with applications involving telecommunications where the level of C-message noise must be very low. Additionally, the current ripple of the boost inductor is small with the present invention. This is advantageous because less EMI filtering is required to meet the above-mentioned IEC standard.
Because only one feedback control loop is required in the present invention to operate the switches within the power converters of the power supply system, the circuitry is simplified. Moreover, the response of the power supply system output voltage is fast because the feedback loop can be selected to have a wide bandwidth.
Additionally, because a regulated output voltage may be obtained for a universal input voltage range (e.g., 86VRMS to 265VRMS), an auto range circuit is not required.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and preferred embodiments, the accompanying drawings, and the appended claims.